Keywords: modal pushover analysis, bridge, stiffness ratio.

When a structure is subjected to overloads such as earthquake-induced strong ground motions, nonlinear structural response with large deformation of the primary members develops and it can lead to the collapse of the structure. To ensure the safety of a structure during strong earthquake loads, accordingly, an accurate estimation of the nonlinear response according to the loading history and a reasonable assessment of the seismic performance are essential. As a part of these efforts, adoption of a capacity-based design in the seismic design code of the ATC has been enlarged, and the ATC-40 [1] and FEMA-273 [2] documents contain simplified nonlinear analysis procedures to determine the displacement demand imposed on a building expected to deform inelastically.

Nevertheless previously introduced capacity-demand methods, such as the equivalent single degree of freedom method , have given reasonable results to estimate building structure behaviour under earthquake loads. However for bridge structures such previously introduced capacity demand methods are limitated because bridge structures are affected by higher modes than building structures.

Earlier a simple but effective analysis procedure, called the improved modal pushover analysis (IMPA), to estimate seismic capacities of multi-span continuous bridge structures was proposed on the basis of modal pushover analysis considering all the dynamic modes of a structure. Nonetheless the proposed method has never approved the effectiveness and applicability to multi-span continuous bridges with large differences in the length of adjacent piers.

So this paper, accordingly, concentrates on a parametric study to verify the efficiency of the IMPA through a correlation study between many analytical models such as the equivalent single degree of freedom method and modal pushover analysis proposed in the previous studies using 48 different bridges which have different deck-pier stiffness ratios and bridge shapes.

This paper shows the maximum and average error rate through different deck-pier stiffness ratios. Also in this paper an acceptable stiffness ratio between deck and pier which makes it possible to use IMPA in predicting the seismic response of a bridge is proposed on the basis of the numerical results obtained.

This paper also provides IMPA maximum and average error rates through different deck-pier stiffness ratios.

1

Applied Technology Council, "Seismic evaluation and retrofit of concrete buildings", Report ATC 40, November, 1996.

2

Federal Emergency Management Agency, "NEHRP guidelines for the seismic rehabilitation of buildings", FEMA 273; and "NHERP commentary on the guidelines for the seismic rehabilitation of buildings", FEMA 274; October, 1997.

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